The present invention relates to large lipid vesicular membrane structures having single bilayers surrounding aqueous interiors. More specifically, the present invention relates to unilamellar lipid vesicles having large encapsulation efficiency and captured volume. The large unilamellar vesicles of the invention are useful, for example, as carriers for hydrophilic, biologically active molecules.
Liposomes or lipid vesicles composed of lipid bilayers enclosing an interior aqueous volume are known to be useful as delivery systems for, or carriers of, various substances. There are three general types of liposomes: multilamellar vesicles (MLVs) composed of more than one concentric lipid bilayer separated by a multiplicity of enclosed volumes; small unilamellar vesicles (SUVs) composed of a single lipid bilayer enclosing a single interior volume and having a diameter of less than 0.2.mu.; and large unilamellar vesicles (LUVs) composed of a single bilayer enclosing a single interior volume and having a diameter of greater than 0.450.mu., and preferably greater than 1.0.mu..
Lipid vesicles can be characterized by a number of functional properties. Properties of importance include captured volume, a measure of the amount of solvent trapped within the vesicles; and encapsulation efficiency (E.sub.e), a measure of the amount of the material to be encapsulated enclosed entirely within the vesicle's internal volume. The captured volume is defined as the concentration of the aqueous fraction inside the vesicle divided by the concentration of lipid in the vesicle, normally expressed as 1/mole lipid. The encapsulation efficiency is defined by the equation E.sub.e =C'/C x 1/C.sub.L, where C' is the final molar concentration of the molecule to be encapsulated within the lipid vesicle, C is the initial molar concentration of the molecule in its solvent, and C.sub.L is the concentration of lipid in the vesicle.
For some uses, e.g., carrying drugs to a specific tissue without dose-related toxicity problems, the vesicles which encapsulate or trap the largest amount of a desired material, and which, upon injection, are the most successful in reaching the targeted tissue are the most valuable. Each type of lipid vesicle is well suited for a different purpose. For example, MLVs are particularly useful for capturing lipophilic materials because of their small aqueous lipid ratio, whereas SUVs, although having low encapsulation efficiencies have the widest tissue access by virtue of their size. LUVs, although very poor in encapsulating hydrophobic or lipophilic materials because of their large aqueous to lipid ratio, have large captured volumes (approximately 35 1/mole lipid) and high encapsulation efficiencies for hydrophilic materials (40-50%). Therefore, LUVs are often the vehicles of choice for transporting hydrophilic, biologically active molecules.
In an effort to create stable, nonantigenic structures having properties similar to natural phospholipid membranes, liposomes have traditionally been synthesized primarily from phospholipids. However, construction of lipid vesicles solely from phospholipids does not accurately recreate the native environment of a membrane since natural membranes also contain proteins and glycoproteins. In addition, phospholipid structures are notoriously unstable in vivo. Factors responsible for this instability include degradation by phospholipases and plasma lipoproteins such as HDL, and by autocatalyzed peroxidation. LUVs, in particular tend to be particularly susceptible to such lytic digestion and peroxidation since they have only a single lipid bilayer surrounding a large aqueous interior.
As well as having the aforementioned stability problems, phospholipids are expensive, making their cost in large scale preparation of liposome operations prohibitive.
In an attempt to solve some of the problems associated with the use of phospholipids in the construction of liposomes, it has been demonstrated that lipid membrane structures, and primarily MLVs, can be prepared from other amphiphilic molecules including fatty acids, dialkyl dimethyl ammonium compounds, and dialkyl amphiphiles with nonionic and zwitterionic head groups, L'Oreal has disclosed the construction of multilamellar vesicles out of synthetic amphiphiles such as alkyl esters of certain polyglycerols and polyoxyethylene, while others have attempted the synthesis of similar vesicles from aliphatic lipids and digalactosyl diglyceride.
Although these efforts have met with varying degrees of success, none have solved all of the foregoing problems.
Accordingly, it is an object of the present invention to provide structurally stable LUVs with high captured volume and high encapsulation efficiency for hydrophilic molecules.
It is an additional object of the invention to provide LUVs which are constructed from lipids which are inexpensive, readily available, biocompatible, and biodegradable.
It is another object of the invention to provide LUVs composed of synthetic, non-phospholipid amphiphilic molecules which are stable, and which are capable of encapsulating hydrophilic molecules.
It is yet another object of the invention to provide LUVs which are capable of encapsulating and transporting hydrophilic molecules.
It is a further object of the invention to provide a delivery system for hydrophilic molecules consisting of an LUV comprising a surfactant, a sterol, a charge-producing amphiphile, and a targeting molecule.
These and other objects and features of the invention will be apparent from the detailed description and claims which follow.